This project will compare and contrast the molecular biology of human rhinovirus C (HRV-C) to viruses in the better known HRV-A and HRV-B species. HRV are canonically linked to the common cold, but more recent studies have clearly established that these viruses, and especially HRV-C, are closely linked to lower respiratory infections and wheezing illnesses. It's the job of the first viral proteins produced in the first 2-3 hrs of infection, in the first infected cells, to shutoff essential sentinel host response systems. Evolution has superbly refined HRV proteases (2Apro and 3Cpro) with species and strain-specific preferences for key cellular substrates. We hypothesize that the magnitude of the immunological alarms set off by any individual HRV have at their origin, the efficacy with which 2Apro does its jobs in the first round(s) of infection. The proposed experiments will document for the first time, the 2Apro genotype diversity within and among the 3 HRV species, and the consequences of that diversity at the biochemical and cellular levels. It will exploit our unique, new-found ability to grow HRV-Cs to identify the cellular receptor for these viruses and expose their comparative molecular secrets. Given that many natural isolates of the HRV-C are mapped recombinants (with the HRV-A) in the 2Apro region, it is paramount to understand what, how and why this protein contributes to individual virus-host interfaces. Specifically, this project proposes three aims: 1) to identify the HRV-C cellular receptor and the natural cell type(s) infected by this virus; 2) to determine the biochemical consequences of virus-specific 2Apro sequence variation by studying the activities of comparable recombinant 2Apro towards cell proteins crucial for virus-induced shutoff of host nucleocytoplasmic trafficking; and 3) using full-length recombinant, chimeric viruses derived from HRV-A, HRV-B, and HRV-C cDNAs, test whether species- and strain-specific 2Apro genes and/or 5' untranslated regions (UTR) influence viral replication and immune responses. These experiments will utilize epithelial cells and clinical HRV isolates collected in Project I, and there will be extensive intellectual and experimental interactions with Project 3 to determine the role of 2Apro on HRV growth in single cells and cell-to-cell transmission.